<meta charset="utf-8">
<title>checkbox</title>
<script>
function validate(){
	var elm = document.reservation.city;  	
	for (var index=0; index < elm.length ; index++)		
	   	if(elm[index].checked) return true;
	return false; 
}
</script>

<form name="reservation" action="next.html" onsubmit="return validate()">
   	도시를 선택하세요: 
 	<input type="checkbox" name="city" value="seoul">서울
 	<input type="checkbox" name="city" value="pusan">부산
 	<input type="checkbox" name="city" value="jeju">제주<br>
 	<input type="submit">
</form>